Methods for treating an inorganic slurry

ABSTRACT

A method for treating an inorganic slurry to preserve the slurry against bacterial contamination, including (I) providing a slurry in a substantially homogeneous phase, and then (II) adding to the slurry an effective amount of a composition containing: (a) a tetrakis(hydroxyorgano)phosphonium salt selected from the group consisting of tetrakis (hydroxymethyl) phosphonium sulphate, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium phosphate, tetrakis (hydroxymethyl) phosphonium nitrate and tetrakis (hydroxymethyl) phosphonium oxalate; and (b) a dispersant selected from the group consisting of (i) a phosphonated compound containing at least one tertiary nitrogen atom and (ii) a homopolymer of an unsaturated acid; and (III) preserving the slurry against bacterial contamination, whilst avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis (hydroxyorgano) phosphonium salt.

This invention relates to a composition for treating inorganic slurries and to a method of treating inorganic slurries with the aforesaid composition so as to maintain the slurries in a substantially homogeneous phase.

The present invention will be described herein with particular reference to calcium carbonate-based slurries, especially those used in paper-making processes, although it is not to be construed as being limited thereto.

Most inorganic slurries contain about 70% to 80% by weight of solids. Many inorganic slurries (particularly those based on calcium carbonate) are known to be susceptible to bacterial contamination and it has been the practice to add one or more biocidally-active materials to the slurries in order to minimise such contamination.

Phosphorus-containing compounds, in particular tetrakis(hydroxyorgano)phosphonium salts (THP⁺ salts) are known to be effective biocides. Experimental work carried out by the applicants has shown, for example, that the addition of a solution of tetrakis(hydroxymethyl)phosphonium sulphate (THPS) to a calcium carbonate-based slurry can give rise to a reduction in bacterial count of 10⁴ in 2 hours.

However, it is also known that addition of THPS alone to a slurry results in instantaneous heterogeneous thickening and aggregation of the slurry.

The applicants have found that the use of a composition comprising a THP⁺ salt and a dispersant will provide continuing preservation against bacterial contamination, while at the same time maintaining the slurry in a substantially homogeneous phase.

Accordingly, in a first aspect, the present invention provides a composition for treating an inorganic slurry, the composition comprising:

(a) a tetrakis(hydroxyorgano)phosphonium salt (hereinafter THP⁺ salt);

and

(b) a dispersant selected from the group consisting of:

-   -   (i) phosphonated compounds containing at least one tertiary         nitrogen atom;     -   (ii) phosphonated oligomers of unsaturated acids;     -   (iii) homopolymers of unsaturated acids;

and (iv) polyphosphates.

In accordance with the present invention, the THP⁺ salt is preferably tetrakis(hydroxymethyl)phosphonium sulphate.

Alternatively, the THP⁺ salt may be tetrakis(hydroxymethyl)phosphonium chloride, phosphate, nitrate or oxalate.

A preferred example of a dispersant of the type (b)(i) is a compound having one tertiary nitrogen atom, such as a sodium salt of nitrilo-tris(methylene phosphate), particularly the tetra-sodium salt.

Preferred examples of dispersants of the type (b)(ii) include those oligomers having the general H(CH₂OM.CH₂OM)_(n)PO₃M₂, wherein M is a cationic species such that the oligomer is soluble in water and n is a number greater than 1.

Other suitable oligomers are disclosed in the applicant's European Patent Specification 0 491 391.

A preferred example of a dispersant of the type (b)(iii) is a homopolymer of acrylic acid, especially a homopolymer having a molecular weight in the range 2000 to 5000.

Preferred examples of dispersants of the type b(iv) include sodium tripolyphosphate.

In a second aspect, the present invention provides a method of treating an inorganic slurry to maintain the slurry in a substantially homogeneous phase, the method comprising the addition to the slurry of an effective amount of a composition according to the first aspect of the present invention.

The inorganic slurry may, for example, comprise a calcium carbonate-based slurry.

Alternatively, the inorganic slurry may comprise a pigment slurry, a clay slurry or a cement slurry.

Preferably, the ratio of THP⁺ salt to dispersant in the composition is about 2:1 (as active ingredients).

Suitably, the composition may be added to the slurry in an amount in the range 10 ppm to 1000 ppm (by weight of the slurry), for example about 750 ppm (by weight of the slurry).

The present invention will be illustrated by way of the following examples.

In the examples, a 75% calcium carbonate slurry (commercially known as Setacarb) was treated with:

EXAMPLE 1

THP⁺ salt alone.

EXAMPLE 2

THP⁺ salt and dispersant of type (b)(i).

EXAMPLE 3

THP⁺ salt and dispersant of type (b)(ii).

The amounts of each additive used, and the results, are given in the TABLE below.

TABLE Example THP⁺ salt Dispersant No. (ppm) (ppm) Result 1 (a) THPS (b) (nil) Instant heterogeneous 750 ppm thickening 2 (a) THPS (b) (i) No thickening 750 ppm 375 ppm 3 (a) THPS (b) (ii) No thickening 750 ppm 375 ppm Notes to TABLE (a) An aqueous solution of tetrakis(hydroxymethyl)pnosphonium sulphate (75% a.i.), available as TOLCIDE ®-PS75. (b) (i) An aqueous solution of the tetra sodium salt of nitrilo-tris(methylene phosphonic acid), available as BRIQUEST ® 301-32S. (b) (ii) A homopolymer of polyacrylic acid, having a molecular weight in the range 2000-5000 and available as BEVALOID ®211. 

1. A method for treating an inorganic slurry to preserve the slurry against bacterial contamination, comprising the steps of: (I) providing a slurry in a substantially homogeneous phase, and then (II) adding to the slurry an effective amount of a composition comprising: (a) a tetrakis(hydroxyorgano)phosphonium salt selected from the group consisting of tetrakis (hydroxymethyl) phosphonium sulphate, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium phosphate, tetrakis (hydroxymethyl) phosphonium nitrate and tetrakis (hydroxymethyl) phosphonium oxalate; and (b) a dispersant selected from the group consisting of (i) a phosphonated compound containing at least one tertiary nitrogen atom and (ii) a homopolymer of an unsaturated acid; and (III) preserving the slurry against bacterial contamination, whilst avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis(hydroxyorgano)phosphonium salt.
 2. The method according to claim 1, wherein the tetrakis (hydroxyorgano)phosphonium salt is tetrakis(hydroxymethyl) phosphonium sulphate.
 3. The method according to claim 1, wherein the tetrakis (hydroxyorgano)phosphonium salt is tetrakis(hydroxymethyl) phosphonium chloride, phosphate, nitrate or oxalate.
 4. The method according to claim 1, wherein the dispersant (b(i)) is a phosphonated compound containing one tertiary nitrogen atom.
 5. The method according to claim 4, wherein the dispersant (b(i)) is a sodium salt of nitrilo-tris(methylene phosphonate).
 6. The method according to claim 5, wherein the salt is the tetra-sodium salt.
 7. The method according to claim 1, wherein the dispersant (b(ii)) is a homopolymer of acrylic acid.
 8. The method according to claim 7, wherein the homopolymer has a molecular weight in the range 2000 to
 5000. 9. The method according to claim 1, wherein the ratio of the tetrakis(hydroxyorgano)phosphonium salt to the dispersant in the composition is about 2:1, as active ingredients.
 10. The method according to claim 1, wherein the composition is added to the slurry in an amount in the range 10 ppm to 1000 ppm, by weight of the slurry.
 11. The method according to claim 1, wherein the composition is added to the slurry in an amount of about 750 ppm, by weight of the slurry.
 12. The method according to claim 1, wherein the slurry comprises a calcium carbonate-based slurry.
 13. The method according to claim 1, wherein the slurry comprises a pigment slurry, a clay slurry or a cement slurry.
 14. The method of claim 1, wherein the slurry being provided contains 70 to 80% by weight of undissolved suspended solids.
 15. The method of claim 1, wherein the slurry being provided comprises a calcium carbonate slurry that contains 70 to 80% or more, by weight, of undissolved suspended solids.
 16. The method of claim 15, wherein the dispersant is selected from the group consisting of (b)(i) a sodium salt of nitrilo-tris(methylene phosphonate) and (b)(ii) a homopolymer of acrylic acid having a molecular weight in the range of 2,000 to 5,000.
 17. The method of claim 16, wherein the composition is added to the slurry in an amount in the range 10 ppm to 1000 ppm, by weight of the slurry.
 18. A method for treating an inorganic slurry to preserve the slurry against bacterial contamination, comprising the steps of: (I) providing a slurry in a substantially homogeneous phase, and then (II) adding to the slurry an effective amount of a composition comprising: (a) a tetrakis (hydroxyorgano) phosphonium salt selected from the group consisting of tetrakis (hydroxymethyl) phosphonium sulphate, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium phosphate, tetrakis (hydroxymethyl) phosphonium nitrate and tetrakis (hydroxymethyl) phosphonium oxalate; and (b) a dispersant which is the tetra sodium salt of nitrilo-tris (methylene phosphonate); and (III) preserving the slurry against bacterial contamination, whilst avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis (hydroxyorgano) phosphonium salt.
 19. A method for treating an inorganic slurry to preserve the slurry against bacterial contamination, comprising the steps of: (I) providing a slurry in a substantially homogeneous phase, and then (II) adding to the slurry an effective amount of a composition comprising: (a) a tetrakis (hydroxyorgano) phosphonium salt selected from the group consisting of tetrakis (hydroxymethyl) phosphonium sulphate, tetrakis (hydroxymethyl) phosphonium chloride, tetrakis (hydroxymethyl) phosphonium phosphate, tetrakis (hydroxymethyl) phosphonium nitrate and tetrakis (hydroxymethyl) phosphonium oxalate; and (b) a dispersant which is a homopolymer of acrylic acid, the homopolymer having a molecular weight in the range of 2,000 to 5,000, and (III) preserving the slurry against bacterial contamination, whilst avoiding instantaneous heterogeneous thickening of the slurry due to the tetrakis (hydroxyorgano) phosphonium salt. 